Dead cat bounces

Hello many times I have red this : ''Deadcats are preceded by heavy sell volume''
My question is: how can it be that there is heavy ''sell volume'' when the volume should be ''neutral'' (for each sell there is a buy)
What is meant with ''sell volume'' ?

Also: Are deadcat bounces preceded by higher volume if compared to the volume of the bullish upword trend tht then caused the deadcat bounce)?

'Heavy sell volume' usually means 'high volume on bearish bars' - i.e. the price was going down and the volumes were high. Whether it is a good predictor for an upcoming "dead cat bounce", I am not sure.